Liquid crystal materials have been known for a long time, but recently have been studied intensively as it has become apparent that their unique properties can be put to a variety of practical uses. The liquid crystal materials possess the fluidity of liquids together with the optical anisotropy of solids. The anisotropy which, in certain varieties of the materials, can produce optical rotation, is susceptible to alteration by imposition of electric or magnetic fields. Display devices are known which take advantage of this change in optical rotatory power as the result of imposition of a field.
In conventional liquid crystal displays, light-scattering is produced in nematic liquid crystals by means of transport of ions under the influence of an electric field; the liquid crystals used for this purpose are termed nematic and are of negative dielectric anisotropy.
Liquid crystals are classified as smectic, cholesteric and nematic. Nematic liquid crystals utilized in electro-optical displays are further subdivided into those having positive and those having negative dielectric anisotropy. Where the nematic liquid crystal has positive dielectric anisotropy, the direction of the dipole moment of the liquid crystal molecule is along the macro-axis of the molecule; where the nematic liquid crystal is of negative dielectric anisotropy, the dipole moment is at right-angles to the macro-axis of the molecule. In the conventional liquid crystal display device, dynamic scattering caused by movement of ions under the influence of an electric field disturbs the ordered arrangement of the liquid crystal molecules and results in light-scattering. When a cell containing a liquid crystal of negative dielectric anisotropy is backed up with a black background, and the cell is viewed by ambient light, then imposition of an electric field on selected portions of the cell causes these portions to appear bright against a dark background. For this type of display, the more negative the dielectric anisotropy of the liquid crystal compound, the more effective the display will be.
The following table presents examples of liquid crystal compounds of both positive and negative dielectric anisotropy.
______________________________________ NEMATIC LIQUID CRYSTAL MATERIALS Type of Dielectric Anisotropy Liquid Crystal Compounds ______________________________________ Negative p-azoxyanisole (PAA) Anisylidene-p-amylphenylacetate (APAPA) p-methoxybenzylidenebutylaniline (MBBA) Positive p-butoxybenzylidene- p'-aminobenzonitrile p-ethoxybenzylidene- p'-aminobenzonitrile ______________________________________
The conventional method of using nematic liquid crystal materials having positive dielectric anisotropy is to place them between transparent plates each having at least one transparent electrode on the interior surface thereof, the plates each having been oriented as by unidirectionally rubbing with a material such as gauze, cotton-wool or the like. Other methods of orienting the surface are also well-known. The plates are then mounted so that the directions of orientation are at right-angles to each other. Under such conditions, when liquid crystal material of positive dielectric anisotropy is placed in the cell formed by the opposed plates, the molecules immediately adjacent the plates align themselves with the orientation directions. The molecules intermediate the plates align themselves along a helix making a quarter turn from one plate to the other. As a result, linearly-polarized light passing through the cell is rotated through an angle of 90.degree.. However, if an electric field of sufficient strength is imposed across the cell in a direction parallel to the axis of the helix, the molecules align themselves with the electric field and the optical rotatory power of the cell disappears.
As is evident, to make use of the phenomenon of the dependence of optical rotatory power on the presence or absence of an electric field, it is necessary that polarizer and analyzer plates be used in conjunction with the cell and with the liquid crystal material therein. Such polarizer and analyzer plates must absorb a substantial proportion of the light falling thereon, especially when the system is used as a display device which must be traversed twice by ambient light, as is the case when the display is used by reflected light. The absorption of the light degrades both the overall intensity of the display as well as the contrast thereof. Moreover, since a mirror must be used for reflection of the light back through the cell, it is generally found that a double image is generated, the second image being fainter than the principal image. The double image arises from the fact that the mirror is a separate element in the assembly. It is evident, therefore, that a cell construction which provides a more legible display over a wider range of ambient light intensities, and which is free of a double image would be of substantial advantage.